From my experience, it depends very much on the instrument.
I experience the very same thing on drums as you describe, I play my kit at home, then play someone else's (who is a much better and more developed drummer than I am) and I suddenly feel much more advanced playing on his beautiful snare, shimmering cymbals and booming toms.
However, playing guitars are different. Because guitar playing is much more personal to you, you need to get used to the feel on the instrument, as you are having a lot more precise contact with it. Therefore practising on different instruments can have an effect on your playing (or that's what I've found).
Take my situation. I appreciate acoustic and electric guitars are in no way the same, both stylistically and musically, but the differing feel and set-up of the different instruments helps me to illustrate my point. I have an acoustic in my room, and play electric when I have time, in the shed outside (stops my family complaining). My acoustic is a Yamaha, with heavy strings and a moderately high action. My electric has a much lower action, with lighter D'addarios. I play my acoustic whenever I am at a loose end in my room, practising solos, tricky songs and riffs on it (often ones that were written for an electric). I them play them on my Tele, and find I have a lot more difficulty playing them at correct speed without mistakes.
I put this down to my fingers being used to applying more pressure and firmness to the strings on my acoustic; when I transfer to electric, that pressure and firmness is not required as much, therefore my fingers slip, and I'm feeling tense when playing.
As a result, I'm currently looking into purchasing a semi-hollowbody guitar, and fitting heavier strings, to give it more the feel of an acoustic.
Regarding your original question, I think going from a bad instrument to a good one is different from going from a good one to a bad. Learning on a difficult one means you have to develop habits in order to play it well, which may not transfer over to a better set-up instrument very well.
I find on bass, it's somewhere in-between, I can play other people's instruments fine, but I prefer playing my own, feeling much more at home, and therefore my playing improves. As Stephen says, it's your baby.
Because on guitars, there are so many things that can vary; action, string gauge, fretboard feel, neck radius, string spacing, therefore when you get settled on one particular combination, you have to deal with adjusting to another combination as well as playing your stuff on the new instrument, hence your playing will change (and in my case suffer!).
Ok, essay over, just a collection of my thoughts, as I am considering this topic a lot more recently due to my current guitar plans that I mentioned. Thanks for listening!